This invention relates to digital image processing and in particular to automatically processing a digital image to produce a visual color reproduction of a scene. More specifically, the invention relates to a visual color reproduction of a scene having preferred color reproduction.
Color image reproduction methods and systems known in the art capture images on image-receptive media, which can be stored in analog or digital form, and then output as a visual reproduction. For example, color images may be captured on photographic negative film and then reproduced optically on photographic paper. Images can also be captured on positive photographic media, and then viewed directly, or copied onto other transparent and reflective media. In addition, color negative films, transparency films or reflective prints can be scanned for input to digital imaging systems. Subsequently, digital color and tone manipulations can be applied to the digital picture element (pixel) values in order to produce the best possible reproduction for the intended output device and medium, and the resulting images can be viewed on monitors, printed on silver halide photographic paper or on other reflective media using inkjet, dye sublimation or electrophotographic printers. Digital images can also be encoded in a defined color space and stored on various media, e.g. Kodak Photo CD, Kodak Picture Disk or CD, at any point in this sequence for future processing. In other cases, color images can be captured by electronic devices, such as video or still CCD cameras, and viewed on monitors or printed using inkjet or dye sublimation thermal printers.
In each case previously cited, these systems are subjected to customer satisfaction criteria and may or may not embody digital tone reproduction manipulation or some form of color enhancement. The systems mentioned above are just some examples of color image reproduction systems.
It is well known in the art, that the best reproductions of original scenes do not constitute a 1:1 mapping of scene colorimetry. For example, the correct scaling of lightness and chroma values depends on the viewing conditions of the original scene and the reproduction. For the purpose of this discussion, viewing conditions are defined as the overall luminance level of the scene or reproduction, the relative brightness of the surround, the state of chromatic adaptation of the observer and the amount of stray light (flare) present. Equivalent color has been defined, as a reproduction, in which the chromaticities, relative luminances and absolute luminances are such that, when seen in the picture-viewing conditions, they have the same appearance as the original scene. This type of match is addressed by color appearance models. It has been argued that equivalent color reproduction produces high quality images.
There is another type of color reproduction that can enhance images beyond equivalent reproduction. Preferred color reproduction is defined as a reproduction in which the colors depart from equality of appearance to those of the original, either absolutely or relative to white, in order to give a more pleasing result to the viewer. Some preferred color enhancements are based on the concept of memory colors. Research has shown, that our memory of certain colors, for example skin colors, foliage and blue sky, deviates from the actual color. Memory colors often have different hues and enhanced colorfulness compared with the actual colors. There is evidence that viewers prefer reproductions that are closer to the memory color than to the actual color. Several researchers have tried to obtain optimum positions for these colors in controlled psychophysical experiments. However, the results often contradict each other, and it has been shown that color preferences may change over time as systems with larger color gamuts become available. The concept of memory colors has never been systematically incorporated into the design of color reproduction systems. While the principles of preferred color reproduction, including the importance of hue reproduction and memory colors, were recently summarized by Hunt in a general fashion (R. W. G. Hunt, xe2x80x9cHow To Make Pictures and Please Peoplexe2x80x9d, The Seventh Color Imaging Conference, ISandT, Springfield, Va., 1999), it is not obvious how to make images according to these principles. Our experience has shown that it is impossible to produce images that embody all the principles of preferred color reproduction using conventional silver halide film/paper systems.
Current optical and digital photofinishing systems produce hues of reproduced colors that change as a function of lightness and chroma, thus giving the reproductions a somewhat unnatural appearance. FIG. 1 shows an example of the hue reproduction capabilities of a current consumer color negative/positive system in terms of a CIELAB a*/b* plot. For demonstration purposes the CIE 1976 a,b chroma, C*ab, was maintained at the original color position. The tails of the arrows denote the original color while the heads of the arrows (symbols) show the reproduced color. In this diagram, colors of constant CIE 1976 a,b hue angle, hab, fall along lines that emanate from the origin (a*=0, b*=0). The abscissa approximately corresponds to the green-red axis, while the ordinate represents the blue-yellow axis. Colors of constant CIE 1976 a,b chroma are represented by concentric circles around the origin. FIG. 1 shows that hues of colors of similar original hue angles may change in opposite directions. Furthermore, hue angle errors of saturated (high chroma) colors are often so large, that a reproduced color may cross a color name boundary. FIG. 1 for example suggests that saturated greens might be reproduced yellow.
One of the important criteria for viewer satisfaction in photographic reproductions is the correspondence between the color stimuli in the original scene compared to those of the reproduction. We find that viewers generally prefer to have high quality images with pleasing tone reproduction, pleasing hues, and high colorfulness while maintaining good skin tone. Technological advances have been made over the years in photographic films by improving spectral sensitivities, incorporating more chemical enhancement, in photographic papers by increasing the paper contrast, and in the whole system by co-optimizing film and paper spectral sensitivities and dyes. Some current methods for making color reproductions produce fairly bright colors and offer reasonable skin tone reproduction; however, there have been limitations on the extent to which color enhancement can be employed. Conventional silver halide photographic systems are subject to limitations imposed by optically printing one chemically developed material onto another chemically developable material. As a result, we find that they generally do not reproduce the scenes in a way that is most preferred by the viewer.
Aside from color enhancement, the quality of image reproductions is also affected by the tone scale or tone mapping employed to reproduce the density variations that make up an image. It has previously been discovered that the use of a preferential tone scale or mapping as described generally in U.S. Pat. No. 5,300,381, issued Apr. 5, 1994 to Buhr et al., entitled xe2x80x9cColor Image Reproduction of Scenes with Preferential Tone Mapping,xe2x80x9d can be utilized to provide a reproduced image that is perceived by the viewer to be a reproduction of the original scene preferred to that previously obtainable. Buhr et al. also provided a solution to the problem of producing pleasing skin tones in combination with high color saturation, as described in U.S. Pat. No. 5,528,339, issued Jun. 18, 1996, entitled xe2x80x9cColor Image Reproduction of Scenes with Color Enhancement and Preferential Tone Mapping.xe2x80x9d
The prior improvement in tone mapping and color enhancement has provided a degree of preferred reproduction of color images, but the use of tone mapping alone has not enabled the full extent of improvement desired by the viewer, in particular as far as hue reproduction is concerned. Recently, digital printing (e.g. the Digital Minilab Frontier 350 available from the Fuji Photofilm Company USA) and digitally-modified optical-printing (e.g. Agfa MSP DIMAX(copyright) printer available from Agfa A.G.) photofinishing systems have been introduced. These systems have introduced improvements in tone reproduction but have done little to improve color reproduction. Moreover, it has not been fully appreciated that the preferred visual reproduction does not usually correspond to the most calorimetrically accurate rendition. There is a need, therefore, for an improved image processing method that produces improved color reproduction.
The need for improved color reproduction is met according to the present invention by providing a method of automatically processing a digital color image, the digital color image having pixels values from which lightness, chroma, and hue values of the image can be deduced according to a predetermined transform, that includes the steps of: transforming the pixel values to lightness, chroma and hue values; transforming the hue values by consistently and smoothly moving the hue values within a predetermined region of color space toward or away from hues of predetermined preferred colors; and consistently and smoothly shifting the hue values in a predetermined region of color space to avoid predetermined objectionable colors.
The method of the present invention has the advantage that the reproduced images are preferred by viewers over those produced by current color reproduction systems.